This invention relates to ignition apparatus. More particularly, this invention relates to ignition apparatus for a spark-ignition internal combustion engine.
A spark-ignition internal combustion engine conventionally includes a number of spark plugs situated in a cylinder head of the engine for creating a spark in each cylinder thereof. Each spark plug is connected to a respective terminal of a distributor by a respective high tension ignition lead. The phrase “high tension”, is used to differentiate electrical components that are used to conduct charge at comparatively high potential from those components that conduct large at comparatively low potential. In the case of a typical automobile engine, high tension components can have a potential difference thereacross measured in kV, such as 25 kV, whereas low tension components would be raised to a potential of tens of volts. In operation, the distributor connects a high voltage across each ignition lead, and hence the respective spark plug, rapidly in succession. This high voltage is sufficient to produce a discharge arc, that is to say a spark, across a respective air gap of each spark plug. During the short-lived existence of the spark, charge flows in an associated ignition lead. The nature of the spark is that high frequency current exists in the lead. This is sometimes referred to as “high frequency noise” and tends to result in radio frequency radiation being emitted by the ignition lead. This radiation is sometimes referred to as radio frequency interference (RFI) as it may interfere with nearby electrical apparatus and thus be problematic. For example, in the case of an automobile, such radiation may interfere with audio equipment of the automobile, such as an in-car Hi-Fi, and may also interfere with computer processing apparatus of the automobile, such as engine management computers.
In an attempt to address this problem, engine and automobile manufactures have sought to use ignition leads with a high electrical resistance. For example, it has been found that use of ignition leads with a resistance of 16 kΩ/m acts to suppress high frequency noise. A drawback of increasing the resistance of ignition cables, however, is that the intensity of the spark may be reduced, resulting in incomplete combustion which in turn leads to reduced power output and increased emissions from the engine. The heating of the ignition leads brought about by their high resistance may also shorten their useful life. There is therefore a trade-off between high frequency noise suppression on the one hand and engine performance and ignition lead life on the other. In attempt to strike the correct balance, at least some international standards limit the resistance of ignition leads to 16 kΩ/m. Thus, the tendency has been for manufacturers to favour leads of high resistance but which are within this limit, for example leads with a resistance of 16 kΩ/m. Such leads, however, still give rise to the drawbacks set out above.
It is an object of at least one embodiment of this invention to address this problem.
Currently-available ignition cables tend to be of one of three different types of construction. The first type includes a highly electrically conductive metal wire, such as copper, to form an electrically conductive core. The second type includes electrically insulating fibres, such as glass or aramid, that are coated with an electrically conductive compound to form the conductive core. The third type also includes electrically insulating fibres, but these are surrounded by a ferrite layer, with a conducting metal wire being wound helically around both the fibres and the ferrite layer to form a core. The wire can be of Ni—Cr alloy, Cu—Sn alloy or stainless steel.
However, each of these types of construction suffers from drawbacks. Drawbacks of the type of construction that uses a copper core include poor resistance to corrosion and poor high frequency noise suppression, together with the resulting cable being rigid and heavy. The second construction type that includes a core formed of insulating fibres coated in a conductor exhibits the undesirable characteristic of an increasing resistance with use. This leads to a worsening of the problems associated with high resistance as set out above. The ferrite layer of the third construction type has poor mechanical properties and is prone to cracking, especially under dynamically varying and tensile forces.
It is an object of at least one embodiment of this invention to address these problems associated with currently-available cables.